As typically seen in a liquid crystal display device, a thin film transistor formed on a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon. A thin film transistor using amorphous silicon has a low field effect mobility, but can be formed on a large glass substrate. On the other hand, a thin film transistor using crystalline silicon has a high field effect mobility, but cannot always be formed on a large glass substrate because of a need of a crystallization process such as laser annealing.
In view of the foregoing, techniques in which a thin film transistor is manufactured using an oxide semiconductor and applied to electronic devices or optical devices have attracted attention. Examples of the techniques are disclosed in Patent Document 1 and Patent Document 2, where a thin film transistor is manufactured with zinc oxide or an In—Ga—Zn—O based oxide semiconductor used for an oxide semiconductor film and is used as a switching element or the like of an image display device.